Single Vortex Creep Research Articles

Temperature and time scaling of the peak-effect vortex configuration in FeTe0.7Se0.3

An extensive study of the magnetic properties of FeTe$_{0.7}$Se$_{0.3}$ crystals in the superconducting state is presented. We show that weak collective pinning, originating from spatial variations of the charge carrier mean free path ($\delta l$ pinning), rules in this superconductor. Our results are compatible with the nanoscale phase separation observed on this compound and indicate that in spite of the chemical inhomogeneity spatial fluctuations of the critical temperature are not important for pinning. A power law dependence of the magnetization vs time, generally interpreted as signature of single vortex creep regime, is observed in magnetic fields up to $8 ~ T$. For magnetic fields applied along the c axis of the crystal the magnetization curves exhibit a clear peak effect whose position shifts when varying the temperature, following the same dependence as observed in YBa$_2$Cu$_3$O$_{7-\delta}$. The time and temperature dependence of the peak position has been investigated. We observe that the occurrence of the peak at a given magnetic field determines a specific vortex configuration that is independent on the temperature. This result indicates that the influence of the temperature on the vortex-vortex and vortex-defect interactions leading to the peak effect in FeTe$_{0.7}$Se$_{0.3}$ is negligible in the explored range of temperatures.

Flux Creep in the Quasi-1D Superconducting Carbide Sc3CoC4

Abstract The superconducting flux dynamic of the transition metal carbide Sc3CoC4 which exhibits a quasi-one-dimensional structure is studied. Besides zero-field-cooling (zfc), field-cooling (fc) and magnetization measurements, especially flux creep relaxation experiments are performed. The relaxation rates S = d M /dln t are determined at selected temperatures below the transition temperature T c in two magnetic fields of 50 Oe and 100 Oe just above H c1. The resulting supercurrent dependence on the mean activation energy is analyzed according to the collective pinning theory which predicts U ∼ (( j / j c)- μ –1). The calculated μ -values differ in the high and low temperature region. The μ -values below about 2.5 K are ≈ 0.5 - 0.68 depending slightly on the applied magnetic field whereas at higher temperatures the μ -values are ≈ 0.22 - 0.34. These results might indicate a transition between different types of vortex pinning around 2.5 K changing from single vortex creep at higher temperatures to collective creep of vortex bundles at lower temperatures.

Collective dynamics of a highly dilute vortex lattice inYBa2Cu3O7−δthin films

The dynamics of highly dilute vortex ensembles in YBa 2 Cu 3 O 7 - thin films is investigated by low-field (5-90 Oe) ac susceptibility measurements of the dynamical relaxation rate Q. In a film with relatively weak pinning,flux bundling is found to be effective at all temperatures and down to the lowest fields. In a film with stronger pinning, single-vortex creep dominates only for T 40 K. The behavior of highly separated vortices is hence surprisingly collective in YBa 2 Cu 3 O 7 - Φ . Quantum creep is observed below 11 K, and at about 60 K the elastic vortex description breaks down as the importance of plastic creep gradually increases.

Effect of electron irradiation on vortex dynamics inYBa2Cu3O7−δsingle crystals

We report on drastic change of vortex dynamics with increase of quenched disorder: for rather weak disorder we found a single vortex creep regime, which we attribute to a Bragg-glass phase, while for enhanced disorder we found an increase of both the depinning current and activation energy with magnetic field, which we attribute to entangled vortex phase. We also found that introduction of additional defects always increases the depinning current, but it increases activation energy only for elastic vortex creep, while it decreases activation energy for plastic vortex creep.

Pinning modulation of flux–antiflux dynamics

Flux dynamics in the superconducting slab after suddenly changed of the external magnetic field is studied. Different regimes of the flux–antiflux dynamics have been considered. Structural defects strongly influence the flux–antiflux dynamics and result in the creation of long-living spatially inhomogeneous metastable flux–antiflux distributions. In the regime of single vortex creep, universal scaling behavior has been found in the flux–antiflux relaxation dynamics.

On the magnetization relaxation of ring-shaped Tl 2Ba 2CaCu 2O 8 thin films as determined by superconducting quantum interference device measurements

By using a superconducting quantum interference device (SQUID), the temporal relaxation of the magnetization was determined for ring-shaped Tl 2Ba 2CaCu 2O 8 thin films at various temperatures between 10 K and 80 K in magnetic fields ranging from 2 mT to 0.3 T. Based on these data, a detailed analysis has been performed related to the following methods or models: (1) Fitting the data to the thermally activated flux motion and collective pinning model; (2) Applying the Generalized Inversion Scheme to extract the temperature dependence of the unrelaxed critical current density j c( T) and pinning potential U c( T); (3) Testing a modified Maley's method to obtain the current dependent activation energy for flux motion; (4) 2D vortex glass scaling. It is found that, for low fields (2 mT, 10 mT, 40 mT) the experimental data can be described by an elastic flux motion, most probably due to 3D single vortex creep. At higher fields (0.1 T, 0.2 T, 0.3 T), the observed behavior can be interpreted in terms of plastic flux motion which is probably governed by dislocation mediated flux creep. These high field data can also be consistently described by the 2D vortex glass scaling with scaling parameters ν 2D, T 0 and p being consistent with those derived from corresponding transport measurement. Also, results are presented demonstrating the importance of optimizing the scan length of the sample in a moving sample SQUID magnetometer to avoid artifacts.

Comparison of results of different methods for the analysis of flux creep behavior in a melt-texturedYBa2Cu3O7−xsample showing a fishtail effect

Magnetic investigations of the flux creep behavior of a melt-textured ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}x}$ sample with a pronounced fishtail effect are reported. To get information about the influence of oxygen on the pinning behavior, the sample was measured before and after a change in oxygen content by annealing. From the time dependence of the magnetic moment measured at fields of 1, 3, and 5 T and at temperatures between 4 and 72 K, the current density dependence of the effective activation energy was determined and interpreted in terms of collective pinning theory. The results are compared with those obtained from the simpler analysis within Anderson's flux creep theory, as well as with the results of an analysis in terms of activation energy distributions. Finally, measurements of the hysteresis loops recorded with different field sweep rates were analyzed using the ``generalized inversion scheme.'' The nature of the fishtail is discussed in comparison with existing models, which try to explain this effect. From all these analyses, a phase diagram for the pinning in the investigated sample is proposed, which is based on the assumption that two different types of pinning centers exist, which behave in different ways. Those that dominate in the field and temperature regime where no fishtail appears may be denoted as ``background'' pinning centers. They show a three-dimensional behavior with a change from small-flux-bundle pinning to large-flux-bundle pinning at higher temperatures. The pinning behavior in this field and temperature regime is not influenced by a change in the oxygen content. In contrast, the pinning centers that dominate in the field and temperature regime where the fishtail appears have higher activation energies and show a two-dimensional pinning behavior with a change from single vortex creep to collective vortex creep with increasing field and temperature. These pinning centers are strongly influenced by the oxygen content.

Single vortex creep in Tl2Ba2CaCu2O8 epitaxial thin films.

By measuring the magnetization hysteresis loops and the remanent magnetization relaxation for an epitaxial ${\mathrm{Tl}}_{2}$${\mathrm{Ba}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8}$ thin film, we determined the temperature dependence of the experimental critical current density j(T) and the normalized relaxation rate S(T) at ${\mathit{B}}_{\mathit{e}}$=0.4 T. It is shown that the data j(T) and S(T) can be well fitted to the single vortex creep model with only one fitting parameter in each fitting process. The data j(T) and S(T) were then used as the input for the generalized inversion scheme (GIS); the output of the GIS, i.e., the temperature dependence of the true (unrelaxed) critical current density ${\mathit{j}}_{\mathit{c}}$(T) and the pinning potential ${\mathit{U}}_{\mathit{c}}$(T), follows the theoretical expressions predicted by the single-vortex collective pinning model. Finally, the magnetization (and thus the current) dependence of the activation energy U(${\mathit{M}}_{\mathit{T}=0}$, T=0) was determined by using the magnetization relaxation data. Again, the single vortex creep model explains the resulting U(${\mathit{M}}_{\mathit{T}=0}$, T=0 K) relation very well. \textcopyright{} 1996 The American Physical Society.

Phase diagram for flux creep in Bi-2223 Ag-sheathed tapes

From magnetic relaxation measurements on monocore Ag-sheathed Bi-2223 tapes a phase diagram for flux creep is proposed. With increasing field and temperature a phase transition from 2D single vortex creep to 2D collective vortex creep is found. Both the phase transition and the irreversibility line are shifted to higher temperatures for samples with higher transport critical current density.

Evidence of collective flux creep in YBa 2Cu 3O 7−δ thin film

Low electric field E-j s curves, normalized dynamical relaxation rate Q(T)= dln(j s) dln(E) and experimental critical current density j s(E=E c) were measured from 10K to 85K with a torque magnetometer in the field sweeping process. Basing on the Thermally Activated Flux Creep (TAFC) model and with the predetermined value for c=ln(v 0B), we obtained the current dependent μ value directly from the experiment data. The theoretical predictions μ= 3 2 for small bundle, μ= 7 9 for large bundle collective flux creep and μ= 1 7 for single vortex creep were proved by the experiment.